Field of the Invention
The present invention generally relates to quadrature clocks and more specifically to circuits that can improve accuracy of quadrature clock.
Description of Related Art
A clock is a voltage signal that periodically toggles back and forth between a low level and a high level. A quadrature clock is a clock having four phases comprising a first phase, a second phase, a third phase, and a fourth phase. Ideally, the four phases are uniformly spaced in time; that is: the first, the second, and the third phases are ahead of the second, the third, and the fourth phases by 90-degress, or equivalently one quarter of the clock period, respectively. In practice, the four phases may not be uniformly spaced in time. A uniformity in timing of the four phases of a quadrature clock determines an accuracy of the quadrature clock.
A prior art polyphase filter 100, such as that shown in FIG. 1, can be used to improve an accuracy of a quadrature clock. Here, A1, A2, A3, and A4 are four phases of an input quadrature clock; B1, B2, B3, B4 are four phases of an interim quadrature clock; and C1, C2, C3, and C4 are four phases of an output quadrature clock that has a better accuracy than the input quadrature clock. Polyphase filter 100 includes four capacitors 111, 112, 113, and 114, four resistors 121, 122, 123, and 124; and four inverter buffers 131, 132, 133, and 134. Polyphase filter 100 is well known in the prior art and thus not explained in detail here. A drawback of polyphase filter 100 is: in a monolithic integrated circuit embodiment, the four capacitors 111, 112, 113, and 114 and the four resistors 121, 122, 123, and 124 usually occupy a large physical layout area, as modern integrated circuit fabrication processes, such as a CMOS (complementary metal oxide semiconductor) process, are less area efficient for implementing capacitors and/or resistors than for implementing transistors.
What is desired is a circuit that can improve accuracy of quadrature clock, without using capacitors and/or resistors and thus is more area efficient than the prior art polyphase filter 100.